Optical disk recorders, in particular, employ objective lens having very small size and mass. Typically, such objective lenses are slidably mounted on a moveable support such that the beam passing through the objective lens is appropriately focused and positioned with respect to a record member. Generally, the acquisition of focus i.e., the movement of the objective lens to an in-focus position wherein the laser or other light beam is appropriately focused at the recording level, should be reliably accomplished for preventing retrys and delays in using an optical disk recorder. Also the best focus should be quickly and accurately obtained. It is desired also to ensure that the control of the focus acquisition is simplified for reducing costs of the optical system.
One of the problems involved in accurately acquiring focus is that the focus error signal FES does not provide accurate information as to the lens out-of-focus position during initial lens motion toward the focus plane. Further, a rotating optical disk is subject to axial motions (wow) further complicating acquiring a best focus position for the lens. Because of such axial motions, a lens when at its best focus position, in maintaining such best focus moves axially with the optical disk several millimeters. The focus maintenance is achieved using a so-called linear region of the focus error signal about the best focus position. On the other hand, the depth of field of such lens is in the order of a few microns; therefore, it is desired to accurately and quickly detect a near focus condition which enables a focus servo to reliably and quickly establish focus maintaining operations. Such quick focus acquisition preferably should involve but a single reliable detection action which accommodates a large range of axial lens speeds at focus acquisition time.